Fertilisation (DP IB Biology) : Revision Note

Human fertilisation

• Fertilisation can be defined as:

the fusion of a sperm and an egg cell nucleus to give rise to a diploid zygote

• During sexual reproduction many sperm are released, and the sperm cells are attracted towards the egg cell by chemical signals

• When the first sperm cell reaches the egg cell surface, its membrane fuses with the egg cell membrane

• The sperm cell nucleus then enters the egg cell

  • At this point vesicles released from the egg destroy the sperm tail and its mitochondria

• Temporary membranes form around the two haploid sets of chromosomes within the egg cell cytoplasm; the resulting structures are known as pronuclei

  • DNA replication takes place at this stage in preparation for mitosis

• The two haploid pronuclei come together and the temporary membranes break down

• Condensed chromosomes undergo the first mitotic division of the now diploid cell

  • Subsequent mitotic divisions take place to form a blastocyst